RfcD PRIMARY SANDSTONE. 337 



class. When doubts arise on that subject, the r 

 geological position is necessarily, as in^all analo- 

 gous caseg, the only guide. 



The texture, from being finely granular, or 

 arenaceous, becomes gravelly or^coarse, and/ at 

 length, passes to an ordinary 1 'conglomerated 

 structure. That mode of disposition; however, 

 often differs sufficiently from that which Occurs 

 in the secondary sandstones, : to eriable a minera- 

 logist who is acquainted with t>ofir rocks, to re- 

 cognize the difference, etert in hand specimens. 

 Where the fragments are numerous and^ nearly 

 equal in size, they are often united in such a 

 manner by a crystalline agglutination, as to de- 

 ceive ah unpractised eye so iis to pass for granite. 

 Where they are rounded, however, as is most 

 frequently the case, that mistake cannot happen. 

 Moreover, even in the coarsest conglomerates, 

 the fragments are often united by, or imbedded 

 in a crystalline basis like that which forms the 

 finer varieties. It must lastly be remarked on 

 the subject of texture, that single, or widely de- 

 tached fragments, are very commonly found im- 

 bedded in the finer varieties, even throughout a 



